1. Field of the Invention
The present invention relates to an image forming method and apparatus, and more particularly to a method and apparatus for image forming capable of effectively eliminating color displacement by controlling a clock control motor controlled by a command clock signal and a feedback signal, in accordance with a velocity curve.
2. Discussion of the Background
Background image forming apparatuses are commonly known as electrophotographic copying machines, printing machines, facsimile machines, and multi-functional apparatuses having at least two functions of copying, printing and facsimile functions. Some of the background apparatuses use an intermediate transfer method, and some use a direct transfer method.
The background image forming apparatus using the intermediate transfer method is referred to as an “intermediate transfer image forming apparatus”, and transfers an electrostatic latent image formed on a photoconductor onto an intermediate transfer member before transferring the electrostatic latent image onto a recording medium.
The background image forming apparatus using the direct transfer method is referred to as a “direct transfer image forming apparatus”, and directly transfers the electrostatic latent image onto the recording medium which is conveyed by a recording medium bearing member.
In both background image forming apparatuses, the photoconductor is driven by a photoconductor motor to rotate, and the intermediate transfer member and the recording medium bearing member are driven by a drive motor to rotate.
The photoconductor and the intermediate transfer member rotate while they are held in contact to each other, a surface linear velocity of the photoconductor is required to have the same rate as that of the intermediate transfer member. In a case where the photoconductor rotates at a different rate from the intermediate transfer member, a surface of the photoconductor rubs a surface of the intermediate transfer member, hastening their surface wear.
To prevent the wearing of the surfaces, the intermediate transfer image forming apparatus has employed a stepping motor as the photoconductor motor and the drive motor for controlling the number of input pulses of the stepping motor to synchronize the surface linear velocities of the photoconductor and the intermediate transfer member. Also, the direct transfer image forming apparatus has employed the stopping motor for synchronizing the surface linear velocities of the photoconductor and the recording medium bearing member.
The stepping motor, however, generally consumes a large amount of electric power and produces a loud noise. Therefore, a clock control motor such as a direct current (DC) brushless motor is used as an alternative to the stepping motor. The DC brushless motor is controlled by a command clock signal and a feedback signal, and can reduce the power consumption and the loud noise.
The DC brushless motor, however, may vary its rotation speed particularly when it is started and stopped. In a case where the DC brushless motor is used as the photoconductor motor and the drive motor, the surface linear velocity of the photoconductor may be greatly different from that of the intermediate transfer member or that of the recording medium bearing member, which results in significant wear that shortens its life. Consequently, the DC brushless motor has been thought to be unsuitable for the background image forming apparatus.
FIG. 1 shows an example of the command clock signal of the DC brushless motor. The rotation of the DC brushless motor is controlled by the command clock signal having a predetermined number of clock pulses, as shown in FIG. 1, and the feedback signal output from the DC brushless motor.
FIG. 2 shows an example of the surface linear velocities of the photoconductor and the intermediate transfer member when the DC brushless motors are started. The DC brushless motor works as the photoconductor motor which rotates the photoconductor and the drive motor which rotates the intermediate transfer member. The solid line represents the surface linear velocity of the photoconductor, and the alternate long and short dash line represents the surface linear velocity of the intermediate transfer member. The photoconductor motor and the drive motor are controlled by a command clock signal same as the command clock signal shown in FIG. 1. However, when DC brushless motor is started, a significant difference between the surface linear velocity of the photoconductor and the surface linear velocity of the intermediate transfer member may be caused due to a property of the DC brushless motor, loads applied to the photoconductor and the intermediate transfer member, and the difference of the inertias of the photoconductor, as shown in FIG. 2.
FIG. 3 shows a graph of the command clock signal when the DC brushless motor is stopped, and FIG. 4 shows a graph of the surface linear velocity of the photoconductor and the intermediate transfer member when the DC brushless motor is stopped.
When a motor stop signal is issued to stop inputting the command clock signal to the photoconductor motor and the drive motor as shown in FIG. 3, the surface linear velocities of the photoconductor and the intermediate transfer member driven by the DC brushless motor start to decrease down to a level, as shown in FIG. 4, at which the photoconductor and the intermediate transfer member stop as shown in FIG. 4. At this time, a significant difference between the surface linear velocity of the photoconductor and the surface linear velocity of the intermediate transfer member may also be caused due to a property of the DC brushless motor, loads applied to the photoconductor and the intermediate transfer member, and the difference of the inertias of the photoconductor, as indicated by the solid line and the alternate long and short dash line shown in FIG. 4.
As described above, the significant difference between the surface linear velocity of the photoconductor and the surface linear velocity of the intermediate transfer member may cause damages such as scratches on the surfaces thereof and defects such as streaks on an image, resulting in a deterioration of the image. The defects may be observed when the DC brushless motor is used as the drive motor for the recording medium bearing member. Due to the drawbacks as described above, the stepping motor has preferably been used, without solving the problems of high power consumption and loud noise.